Display device

ABSTRACT

The present invention relates to a display device comprising a screen ( 1 ) with a plurality of pixels. Each pixel has a corresponding electron emitting structure ( 8, 9, 11 ), such as a gate-cathode combination. The electrons emitted by each electron emitting structure ( 8, 9, 11 ) are accelerated toward an anode layer ( 12 ) in the screen ( 1 ). The anode layer ( 12 ) is subdivided into a plurality of separate portions ( 12   a, . . .    12   l ), and each such portion has a corresponding current meter ( 15   a, . . .    15   l ) for measuring the portion&#39;s part of the total anode current of the display device. This entails an improved capability of measuring the properties of the individual electron emitting structures, which serves to adjust each electron emitting structure&#39;s signal in order to obtain a more uniform display device.

TECHNICAL FIELD

The present invention relates to a display device, comprising a screenwith a plurality of picture elements, a planar anode electrode, locatedin the screen, a plurality of electron emitting structures, eachcorresponding to a picture element, the electron emitting structuresbeing arranged to emit electrons intended to be accelerated towards theanode, and means for measuring the anode current.

TECHNICAL BACKGROUND

Such a display device is disclosed in EP 1225557, A1. The anode currentmeasuring means allows the properties of each pixel's correspondingelectron emitting element, e.g. its voltage-current characteristics, tobe measured during so-called blanking periods, when the pixels are nototherwise activated. With information regarding the properties of eachpixel's electron source at hand, the signals controlling these sourcesmay be adjusted in order to obtain a more uniform display device, i.e.so that, for a given input signal, all pixels emit light with the samestrength.

A problem with such a display device is that the properties for eachpixel may only be updated at a low rate, since measurements can onlytake place during blanking periods and only for one pixel at a time.This means that pixel property information will not always be up todate, since the properties may change, e.g. with changing operatingtemperature. Moreover, measuring pixel properties during blankingperiods may cause visible disturbances in the display device, since alight signal is produced which does not belong to the received imagesignal.

SUMMARY OF THE INVENTION

An object of the present invention is to wholly or partially obviate theabove problems.

This object is achieved with a display device of the above indicatedtype, wherein the anode electrode is divided into a plurality ofelectrically separate planar anode portions, wherein each anode portioncomprises current measuring means for measuring a portion of a totalanode current. This means that pixel properties may be measured for morethan one pixel at the same time, thus allowing the pixel properties tobe updated more often.

In a preferred embodiment the picture elements are arranged to beactivated in groups, and the anode portions are arranged in such a waythat picture elements, which belong to a given group, correspond todifferent anode portions. This allows the pixel properties to bemeasured during normal displaying, updating all pixels' properties inall display frames and without causing any visual disturbances.

Preferably the picture elements are arranged in lines and columns, thedisplay device being arranged to activate a line at a time, and eachcolumn having a corresponding anode portion in the form of a strip. Thisentails the possibility to update pixel properties during a normal videodisplay process.

Preferably, the display device comprises a memory for storing, for eachpicture element, information relating to the properties of itscorresponding electron emitting structure, which information is basedupon an anode current measured for that picture element.

Preferably, the display device is arranged to use information stored inthis memory for adjusting drive signals for the electron emittingstructures.

In a preferred embodiment, the display device comprises means forintegrating current data measured by said current measuring means. Thisallows the pixel property information to include rise and fall periodsin the current envelops.

Preferably, the display device comprises means for multiplexing currentdata, measured by said current measuring means. This allows a pluralityof current meters to share a single level shifter, which is used toshift the current signal to the voltage level of the electron emittingstructure. This provides for reduced complexity and costs.

Preferably, each current measuring means comprises a current mirror.

In a preferred embodiment, each electron emitting structure comprises agate electrode and a cathode electrode.

In an alternative embodiment, each electron emitting structure comprisesa light source and a portion of a photoelectric layer, the portion ofthe photoelectric layer being arranged to emit electrons whenilluminated by the light source.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a display device according to knownart.

FIG. 2 shows a controllable electron emitting structure, associated witha pixel in a display device.

FIG. 3 illustrates schematically a screen anode arrangement for adisplay device according to known art.

FIG. 4 illustrates schematically a screen anode arrangement for adisplay device according to a preferred embodiment of the invention.

FIG. 5 shows a control arrangement for a display device according to anembodiment of the invention.

FIG. 6 shows a control arrangement for a display device according to analternative embodiment of the invention.

FIG. 7 shows a current mirror arrangement.

FIG. 8 illustrates schematically a level shifting arrangement.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates schematically a display device according to knownart. The display device comprises a screen 1, comprising a large number,e.g. in case of a wxga display 768×1365, of picture elements 2, whichhereinafter are called pixels. The display device may be used forinstance as a computer monitor or a TV. The luminance of the pixels inthe screen are controlled by a line driver 3 and a column driver 4. Byactivating a specific line and a specific column (bold arrows), thedrivers 3, 4 cause a specific pixel 2 in the line-column intersection toemit light. The display device receives a video signal, and a decoder 5generates, from the video signal, horizontal and verticalsynchronization signals (H-SYNC, V-SYNC) and a luminance signal (LUM),which are fed to the drivers 3, 4. By activating the pixels 2 of thescreen 1 an image, corresponding to the received video signal, is thusgenerated. In the display device, each pixel has a correspondingcontrollable electron emitting structure. The figures are of courseschematic e.g. in that they show only 12×12 pixels in order tofacilitate comprehension of the invention. As mentioned above the numberof pixels could be considerably greater.

FIG. 2 shows a controllable electron emitting structure, associated witha pixel in a display device. The structure comprises a cathode electrode8 disposed on a glass substrate 9. On the cathode electrode 8 anemission material 10 is disposed, in contact with the cathode electrode8.

A gate electrode 11 is provided, separated form the cathode by means ofan insulating layer 14. The gate electrode 11 and the insulating layer14 contain holes. At the position of these holes, the gate 11, thecathode 8 and the emission material 10 together constitute acontrollable electron emitting structure. By applying a suitabledifference between potential V_(C) of the cathode 8 and the potential ofthe gate V_(G) (e.g. V_(C)=−30 V and V_(G)=60 V) a local electric fieldis generated near the emission material 10, which causes the emissionmaterial 10 to emit electrons (e). (Note that the cathode itself mayconstitute an emission material in which case no additional layer needbe applied.) The electrons emitted by the controllable electron emissionstructure are accelerated towards an anode 12 in the screen, which anode12 has a significant positive potential (e.g. V_(A)=5 kV). Whenelectrons reach the screen, they hit a phosphorescent layer 13, which asa consequence emits light. A gate electrode 11 may preferably bestrip-shaped and common to all pixels in a line, and is then controlledby a line driver 3. A cathode may preferably be strip-shaped and commonto all pixels in a column, and is then controlled by a column driver 4.

The present invention is also applicable to so-called photo cathodedisplays. Then, each electron emitting structure comprises a lightsource and a portion of a photoelectric layer, the portion of thephotoelectric layer being arranged to emit electrons when illuminated bythe light source.

FIG. 3 illustrates schematically a screen anode arrangement for adisplay device according to known art. The screen 1 comprises acontinuous conductive anode layer 12 which is common to all pixels inthe display device. The anode layer is connected to a voltage source toprovide the anode voltage V_(A). The arrangement further comprisescurrent meter 15 for measuring the anode current I_(A).

When a large number of electron emitting structures of the type shown inFIG. 2 are produced in a manufacturing process, the emissive propertiesof the individual structures will vary over the display. That is, for agiven gate-cathode voltage (in case of an amplitude modulated gate) or agiven pulse ratio (in case of a pulse-width modulated cathode)individual emitting structures will emit different amounts of electrons.This leads to a non-uniform display. Moreover, the properties of theindividual pixels may change also over time, e.g. due to changingambient temperature or aging.

By activating only one pixel and measuring the resulting anode current,properties of the electron emitting structure corresponding to thispixel may be determined and stored in a memory. When the display isused, this information may then be used to adjust the gate or cathodevoltage (or pulse ratio) for individual electron emitting structures inorder to achieve a uniform display.

This measuring takes place in blanking periods when pixels otherwise arenot normally activated. Since the number of pixels is large, theproperties information for each pixels electron emitting structure canbe updated only very seldom and is therefore not always up to date, e.g.when the operating temperature changes.

FIG. 4 illustrates schematically a screen anode arrangement for adisplay device according to a preferred embodiment of the invention.According to the preferred embodiment, the anode layer is structured, soas to form a plurality of electrically separated anode layer portions 12a, 12 b, 12 c, 12 d, etc. Each such portion preferably corresponds to acolumn in the display device. Each portion may comprise an indium tinoxide layer. A current sensor 15 a, 15 b, 15 c, 15 d, etc. is arrangedfor each of the anode layer portions.

Different processes may be used for providing the separated anode layerportions. The layer may be provided as separate portions from the start,e.g. by a printing process. As an alternative, a continuous layer may beprovided, which is subsequently separated into a plurality of portionsin an etching process.

Since the pixels normally are activated a line at a time, the anodecurrents corresponding to each of the pixels in each line may bemeasured individually during regular displaying. This allows the pixelproperty information to be updated each time the pixel is activated.

Note that already by dividing the anode layer in two portions, pixelproperty information may be updated twice as often as compared with acontinuous anode layer, since twice as many pixels can be updated duringeach blanking period.

FIG. 5 shows a control arrangement for a display device according to anembodiment of the invention. The current sensor outputs are generated atthe high anode potential, which means that a level shifter 18 is neededto bring the signal down to the cathode voltage potential. In principle,each current sensor may have its own level shifter, but in order toreduce the complexity and costs a multiplexing arrangement may be usedas illustrated in FIG. 5. In this case four current sensors 15 a, 15 b,etc share a common level shifter 18, and are connected to the levelshifter via a multiplexer 19. The multiplexer receives synchronizinginformation in order to determine which of the inputted signals shouldbe passed on to a memory 20 via the level shifter 18 and an amplifier21. The memory 20 receives corresponding synchronizing information to beable to store the information correctly, i.e. as belonging to aparticular pixel. As will be described later, the current signal value,or another value, calculated based on the current signal value is storedin the memory 20. This value is used by a pulse width (PWM) modulator 22to control the pulse ratio of the cathode voltage.

By using a multiplexer the complexity and the costs of the circuit maythus be reduced. Of course, when for instance four current sensors sharea common level shifter, the property information of each pixel may beupdated with a four times lower frequency, but in many applications thisis allowed. The number of level shifters may therefore be varied betweenone and one for each anode layer portion, depending on the applicationrequirements.

The information stored for each pixel in the memory relates to aproperty value or information that may be used to calculate such avalue. E.g. in case of pulse-width modulation the actual measured anodecurrent I_(meas) may be stored. The cathode pulse ratio T_(pulse) forthat pixel may then be calculated as T_(pulse)=T_(d)*I_(meas)/I_(d),where T_(d) is the ideal pixel pulse ratio for the desired grey scalelevel and I_(d) is the ideal anode current in the high state of thepulse cycle, which current is the same for all pixels. For cases withamplitude modulation, information regarding the emitter signal should bestored together with the measured anode current that is its result, asis recognized by the skilled person.

FIG. 6 shows a control arrangement for a display device according to apreferred alternative embodiment of the invention. Compared with theembodiment in FIG. 5, an integrator 23 is added in this arrangement. Theintegrator serves to make the current signal from each sensor morerepresentative of the electron flow actually received in a pixel. If forinstance PWM-modulation is used, the electron flow varies greatly duringthe activation of a pixel, even if the resulting light emission isrelatively constant. Thus, if the current sensor is sampled at anarbitrarily chosen instant during the activation of a pixel, theresulting current value need not necessarily be representative of theelectron flow actually received at the pixel. The integrator solves thisproblem by providing an output that is representative of the total anodecurrent during the activation of a pixel. In this case, informationregarding the pulse ratio should be stored together with the resultinganode current in order to obtain a description of properties of theindividual emitter element.

In principle, the concept of integrating current measuring may be usedalso at the side of the electron emitting structures. If each cathodecurrent is measured and integrated, the resulting value may be used,together with the cathode voltage or pulse ratio from which it results,to obtain in a similar way information about the pixel properties. Byproviding a current meter at the cathode electrodes and an integratorfor integrating the current obtained for a given cathode voltage orpulse ratio, a property value for the pixel may be obtained, which valuemay be used to adjust the gate voltage or pulse ratio in order to obtaina more uniform display. This feature may thus be used also in displaydevices not employing divided anodes or anode current sensors.

FIG. 7 shows a current mirror arrangement that may be used as a currentmeasuring means. The current mirror comprises first and secondtransistors 26, 27 with interconnected bases, wherein the firsttransistor 26 is diode-coupled. The anode current I_(A) is drawn from acurrent source 28 at a supply voltage V_(sup) and, due to the currentmirror arrangement, the current through a resistor 29 (with resistanceR), connected to the second transistor, will be identical with I_(A).Thus, the voltage V_(out) will be equal to V_(sup)−I_(A)*R. The outputvoltage from the current mirror is thus representative of the anodecurrent. If instead a current output is desired, the resistor may beomitted (R=0).

Other current measuring means are conceivable such as an operationalamplifier in an current to voltage configuration. In general, it isimportant that the input impedance of the current measuring meansmatches with the impedance of the corresponding anode structure.

FIG. 8 illustrates schematically a level shifting arrangement. Thearrangement comprises a primary side part 30, a galvanic isolation part21 and a secondary side part 32. The primary side part 30 at a highpotential of e.g. 5 kV comprises the current measuring means, generatingthe anode current signal and preferably converting it into an AC-signalto be transferred to the secondary side part 32, at the emitter level(at or close to ground level). The secondary side part 32 receives thetransmitted signal and converts it into a format that may be used bycontrol blocks at the emitter level. These parts are separated by thegalvanic isolation part 31, comprising e.g. an isolating amplifier. Theisolation part 31 should withstand the high DC voltage and at the sametime be transparent to the measuring signal. Different types ofcapacitor/transformer combinations, optic components, such asphotodiodes, and other components may be utilized to this end, as iswell known to the skilled person.

In summary, the present invention relates to a display device comprisinga screen with a plurality of pixels. Each pixel has a correspondingelectron emitting structure, such as a gate-cathode combination. Theelectrons emitted by each electron emitting structure are acceleratedtoward an anode layer in the screen. The anode layer is subdivided intoa plurality of separate portions, and each such portion has acorresponding current meter for measuring the portion's part of thetotal anode current of the display device. This entails an improvedcapability of measuring the properties of the individual electronemitting structures, which serves to adjust each electron emittingstructure's signal in order to obtain a more uniform display device.

While the invention has been described in connection with variouspreferred embodiments, it should be understood that the invention shouldnot be construed as being limited to those embodiments. The inventionrather includes all variations which could be made thereto by a skilledperson and within the scope of the appended claims. E.g. instead ofpulse width modulation, as described in the above embodiment, amplitudemodulation of cathode voltage may be used or a combination of pulsewidth modulation and amplitude modulation.

Instead of associating gate electrodes with rows and cathode electrodeswith columns as described above, cathode electrodes may be associatedwith rows and gate electrodes with columns.

The invention is moreover also applicable to so-called under-gateemitters, wherein the gate electrodes are placed beneath the cathodeelectrodes as seen from the anode. Also other gate structures arepossible, such as for instance side-gate emitters.

1. A display device, comprising a screen (1) with a plurality of pictureelements (8, 10, 11), a planar anode electrode (12), located in thescreen, a plurality of electron emitting structures (8, 10, 11), eachcorresponding to a picture element, the electron emitting structures (8,10, 11) being arranged to emit electrons intended to be acceleratedtowards the anode (12), and means for measuring the anode current,characterized in that the anode electrode is divided into a plurality ofelectrically separate planar anode portions (12 a, 12 b, . . . , 12 l),wherein each anode portion comprises current measuring means (15 a, 15b, . . . , 15 l) for measuring a portion of a total anode current.
 2. Adisplay device according to claim 1, wherein the picture elements arearranged to be activated in groups, and wherein the anode portions arearranged in such a way that picture elements, which belong to a givengroup, correspond to different anode portions.
 3. A display deviceaccording to claim 2, wherein the picture elements are arranged in linesand columns, the display device being arranged to activate a line ofpicture elements at a time, and wherein each column has a correspondinganode portion substantially in the form of a strip.
 4. A display deviceaccording to claim 1, comprising a memory (20) for storing for eachpicture element information relating to the properties of thecorresponding electron emitting structures, which information isdependent on an anode current measured for that picture element.
 5. Adisplay device according to claim 4, wherein the display device isarranged to use information stored in the memory (20) for adjustingdrive signals for the electron emitting structures.
 6. A display deviceaccording to claim 1, comprising means (23) for integrating current datameasured by said current measuring means.
 7. A display device accordingto claim 1, comprising means (19) for multiplexing current data,measured by said current measuring means.
 8. A display device accordingto claim 1, wherein each current measuring means comprises a currentmirror.
 9. A display device according to claim 1, wherein each electronemitting structure (8, 10, 11) comprises a gate electrode (11) and acathode electrode (8).
 10. A display device according to claim 1,wherein each electron emitting structure comprises a light source and aportion of a photoelectric layer, the portion of the photoelectric layerbeing arranged to emit electrons when illuminated by the light source.